Neutralizing monoclonal antibodies specific for the fusion (F) glycoprotein of human parainfluenza type 3 virus (PIV3) were used to select neutralization-resistant antigenic variants. Sequence analysis of the F gene of the variants indicated that their resistance to antibody binding, antibody-mediated neutralization or to both was a result of specific amino acid substitutions within the neutralization epitopes of the F1 and F2 subunits. Comparison of the location of PIV3 F neutralization epitopes with the corresponding sites on Newcastle disease virus and Sendai virus indicated that the antigenic organization of the fusion proteins of these other paramyxoviruses is similar. Furthermore, some of the PIV3 F epitopes recognized by syncytium-inhibiting monoclonal antibodies are located in an Fl cysteine cluster region that corresponds to an area of the measles virus F protein involved in fusion activity. More than 90% of infants or young children who underwent primary infection with PIV3 developed antibodies to 4 of the 6 hemagglutinin-neuraminidase (HN) antigenic sites (including 3 of the 4 neutralization sites), whereas the response to F antigenic sites was of lesser magnitude and varied considerably from person to person. Also, the response to both F and HN antigenic sites was suppressed in infants who possessed maternally-derived serum PIV3 antibodies. The restricted immune response to the F glycoprotein during primary infection and the decreased response to both the F and HN glycoproteins in young infants with maternally-derived antibodies may play a role in the susceptibility of the human infant and young child to reinfection with PIV3.